TW201631056A - Conductive paste, electrode, solar cell and method - Google Patents

Abstract

The present invention relates to a conductive paste suitable for use in a solar cell, a method for producing a conductive paste, a light receiving electrode for a solar cell, and a solar cell. The paste comprises a solid portion dispersed in an organic medium, wherein the organic medium comprises a phosphate free acid ammonium salt. The invention further relates to a printing method.

Description

Conductive paste, electrode, solar cell and method

The present invention relates to a conductive paste suitable for use in a solar cell, a method of manufacturing a light receiving surface electrode of a solar cell, a light receiving surface for a solar cell, an organic medium included in a conductive paste suitable for use in a solar cell, And printing methods.

Stencil-printed metal (eg, silver) pastes are conventionally used to form conductive tracks in electronic products and semiconductor applications, such as in solar cells (eg, solar cells). The paste typically comprises a metal (e.g., silver) powder, a mixed oxide (e.g., glass frit) and sometimes one or more additional additives, all dispersed in an organic medium.

Organic media plays an important role in conductive pastes. For example, the nature of organic media is critical to achieving reliable and large-scale paste production, providing pastes with good printability, and providing pastes with good wet strength.

After printing and drying (usually at about 200 ° C), the paste should have good wet strength, allowing the solar cells to be transported and handled without damaging the wire if necessary.

In addition, the ability to print very fine lines of conductive paste is critical to solar cell efficiency. The narrow, well-defined line maximizes the exposure area of the light-receiving surface of the solar cell, thereby increasing its efficiency. However, the shape of the printed line is also important: a larger cross section reduces the resistivity. Therefore, the preferred line shape is to have a narrow high cross section because this situation exposes as many light receiving surfaces as possible while minimizing resistivity.

The ability to print fine lines having the desired cross-sectional shape is clearly dependent on the rheology of the paste, and the nature of the organic medium can be important to provide a paste that exhibits suitable rheological behavior. Of particular importance is the highly structured paste which retains its structure under the high strain conditions observed in screen printing. The inventors have discovered that the paste can lose its structure when it is being screen printed, for example, due to solid/liquid phase separation during printing. This results in the accumulation of the paste or paste component on the lower surface of the screen, which results in the formation of smudges on the edges of the printed line. This smudge is undesirable because it covers a portion of the light receiving surface of the battery but does not contribute to the conductivity of the printed line. It also increases paste loss.

There is still a need for improved conductive pastes that exhibit good stencil printability, such as pastes suitable for fine line printing with minimal smudges at the edge of the line. There is still a need for a paste with improved rheological behavior.

As demonstrated in the examples below, the inventors have unexpectedly discovered that the inclusion of the phosphate free acid ammonium salt in the screen printable paste provides good rheological behavior and good screen printability. Thus, in a general aspect, the present invention relates to the inclusion of a phosphate free acid ammonium salt in a screen printable paste.

In a first preferred aspect, the present invention provides a conductive paste comprising a solid portion dispersed in an organic medium, the solid portion comprising a conductive metal and a mixed oxide, wherein the organic medium comprises a phosphate free acid ammonium salt. Typically, the phosphate free acid ammonium salt is the reaction product of a phosphate free acid and an amine.

Generally, a conductive paste is prepared by mixing together components of a conductive metal, a mixed oxide, and an organic medium in any order. In a second preferred aspect, the present invention provides a method of preparing a conductive paste, wherein the method comprises mixing together components of a conductive metal, a mixed oxide, and an organic medium in any order, and wherein the organic medium comprises phosphoric acid Ester free acid ammonium salt.

In a third preferred aspect, the present invention provides a light receiving for a solar cell An electrode comprising a conductive track on a semiconductor substrate, wherein the conductive track is obtained or obtained by firing a paste according to a first aspect on a semiconductor substrate.

In a fourth preferred aspect, the present invention provides a solar cell comprising a light receiving electrode according to a third aspect.

In another preferred aspect, the invention provides the use of a conductive paste according to a first aspect in the manufacture of a light receiving surface electrode of a solar cell. In still another preferred aspect, the present invention provides the use of a conductive paste according to a first aspect in the manufacture of a solar cell.

In another preferred aspect, the present invention provides an organic medium for a screen printable paste comprising a phosphate free acid ammonium salt dispersed or dissolved in a solvent. The screen printable paste comprising the organic medium of the present invention may be a conductive paste. Alternatively, the organic media of the present invention can be employed in screen printable pastes suitable for use in other applications, such as printing enamel onto glass surfaces such as automotive glass.

Thus, in another aspect, the present invention provides a printing method comprising applying a paste composition to a substrate by screen printing, wherein the paste composition comprises a solid portion dispersed in an organic medium, And wherein the organic medium comprises a phosphate ester free acid ammonium salt. The substrate may be a semiconductor substrate, and the paste composition may be a conductive paste in which the solid portion contains a conductive metal and a mixed oxide. Alternatively, the substrate may be a glass surface such as an automotive windshield, and the conductive paste may be an enamel composition wherein the solid portion comprises a pigment and a mixed oxide.

Figure 1 shows a screen printing line using the paste of Comparative Example 1.

2 to 6 show screen printing lines using the pastes of Examples 1 to 5, respectively.

Figure 7 shows the results of a rheological test as described in the Examples section below.

Fig. 8 shows a screen printing line using the paste of Comparative Example 2.

Figure 9 shows a screen printing line using the paste of Example 6.

Preferred and/or optional features of the invention will now be set forth. Any aspect of the invention may be combined with any other aspect of the invention, unless the context requires otherwise. Any of the preferred and/or optional features of any aspect may be combined with any aspect of the invention, alone or in combination, unless the context requires otherwise.

Conductive paste

Generally, the conductive paste is a conductive paste for a solar cell. Typically, it is a front conductive paste. Typically, it is a stencil printable conductive paste.

(As will be understood by those skilled in the art, the paste form of the conductive paste is not necessarily electrically conductive. Alternatively, the paste is considered to contain a conductive metal and is fired to form a conductive track on the substrate.)

The conductive paste contains a solid portion dispersed in an organic medium. The organic medium can constitute, for example, at least 2 wt%, at least 5 wt%, at least 7 wt%, at least 9 wt%, at least 10 wt%, or at least 11 wt% of the conductive paste. The organic medium may constitute 25 wt% or less, 20 wt% or less, 15 wt% or less, 13 wt% or less, 12 wt% or less, or 10 wt% or less of the conductive paste.

Thus, it will be understood that the solid portion may constitute at least 75 wt%, at least 80 wt%, at least 85 wt%, at least 87 wt%, at least 88 wt%, or at least 90 wt% of the conductive paste. The solid portion may constitute 98 wt% or less, 95 wt% or less, 91 wt% or less, 90 wt% or less, or 89 wt% or less of the conductive paste.

Organic medium

The organic medium comprises a phosphate free acid ammonium salt. Typically, it further comprises an organic solvent in which one or more additives are dissolved or dispersed, as appropriate. As will be readily appreciated by those skilled in the art, the components of the organic medium are typically selected to provide suitable consistency and rheological properties, permitting the printing of the conductive paste onto the semiconductor substrate, and allowing the paste to be transported and stored during transport and storage. stable. As discussed above and as demonstrated in the examples below, the phosphate free acid ammonium salt is provided to provide a paste having rheological properties that make it particularly suitable for screen printing.

Typically, the phosphate free acid ammonium salt is the reaction product of a phosphate free acid and an amine.

Those skilled in the art will understand the term phosphate free acid. As understood by those skilled in the art, the phosphate moiety has the following structure:

The phosphate ester free acid may comprise a single phosphate moiety or may comprise two or more phosphate moieties bonded to form a polyphosphate moiety (wherein the polyphosphate moiety is understood to include two or more (eg, three or more) portions of a phosphate moiety that are linked by a chain (eg, covalently bonded). The phosphate ester comprises a phosphate moiety wherein at least one of the oxygen atoms is attached to the organic moiety. In the phosphate free acid , at least one of the oxygen atoms of the phosphate moiety forms an -OH group. This -OH group is a "free acid" group.

The phosphate free acid used in the present invention may be a phosphate free acid according to formula I below:

Wherein: each R ₁ is independently selected from H and an organic moiety; n is an integer of 1 or more; and at least one R ₁ group is an organic moiety, and at least one R ₁ group is H.

The organic moiety is typically an optionally substituted hydrocarbon moiety, such as a _C5 to _C40 hydrocarbon moiety, as appropriate. For example, the hydrocarbon portion can include at least 5, at least 10, or at least 15 carbon atoms. It may include 40 or fewer, 35 or fewer or 30 or fewer carbon atoms.

When R ₁ is an optionally substituted hydrocarbon moiety, it may be selected from optionally substituted alkyl (including cycloalkyl), alkenyl (including cycloalkenyl), alkynyl, aryl, alkaryl, Aralkyl and alkaryl groups. The hydrocarbon portion can be linear or branched. Preferably, the hydrocarbon moiety is an optionally substituted alkyl, aryl, alkaryl, aralkyl or alkarylalkyl group which may be straight or branched.

When R ₁ is a substituted substituted hydrocarbon moiety, typically one, two, three, four or more hydrogen atoms of the hydrocarbon moiety are replaced by other functional groups. Suitable functional groups include -OH, -SH, -OR ₂ , -SR ₂ , -Hal, -NR ₂ R ₂ , C(O)COR ₂ , -OC(O)R ₂ , -NR ₂ C(O) R ₂ and C(O)NR ₂ R ₂ , wherein each R _{2 is} independently H or a C ₁ to C ₁₀ alkyl or alkenyl group, and wherein each -Hal is independently selected from -F, -Cl and -Br , for example -Cl. For example, suitable substituent functional groups include -OH, -OR ₂ , -Hal, C(O)COR ₂ , -OC(O)R ₂ , -NR ₂ C(O)R ₂ and C(O) NR ₂ R ₂ , wherein each R _{2 is} independently H or C ₁ to C ₁₀ alkyl or alkenyl, and wherein each -Hal is independently selected from -F, -Cl, and -Br, such as -Cl. Particularly suitable substituent functional groups include -OH and -OR ₂ wherein each R _{2 is} independently H or C ₁ to C ₁₀ or C ₁ to C ₆ alkyl or alkenyl (eg, alkyl).

In some embodiments, the hydrocarbon moiety is preferably unsubstituted.

The phosphate free acid typically has a molecular weight of at least 1000 (e.g., at least 1500 or at least 2000). It may have a molecular weight of 10,000 or less, 7000 or less, 5000 or less, or 3000 or less.

As provided above, n is an integer of 1 or greater. For example, n can be at least 2, at least 3, at least 4, or at least 5. It can be 50 or less, 30 or less, 20 or less, 15 or less or 10 or less.

The phosphate free acid typically has an acid number of about 38. For example, it can have an acid number of at least 5, at least 10, at least 15, at least 20, at least 25, or at least 30. It may have an acid number of less than 70, less than 60, less than 55, less than 50, less than 45, or less than 40. As will be understood by those skilled in the art, the acid value is the mass in milligrams of potassium hydroxide (KOH) required to neutralize one gram of the substance in question (in this case, the phosphate free acid).

A particularly suitable phosphate ester free acid is Lubrizol® 2062 (available from Lubrizol®) having a molecular weight of at least 2000, an acid number of about 38, and a mixed alkyl/aryl (eg, alkarylalkyl) side chain (corresponding to In the above formula I, it is not the R ₁ group of H).

Particularly suitable R ₁ groups have a structure according to the following formula II:

Wherein * indicates a point of attachment to the phosphate moiety, and each m is independently an integer from 1 to 20, more preferably from 5 to 10.

As mentioned above, the phosphate free acid ammonium salt is typically the reaction product of a phosphate free acid and an amine. The nature of the amine is not particularly limited in the present invention. The amine is typically a hydrocarbon substituted with one or more amine groups which are optionally further substituted. The amine may contain two or more amine groups (for example, it may be a diamine or a triamine, etc.). Although there is no specific upper limit for the number of amine groups, the amine may include 10 or less, 5 or less or 3 or less amine groups. The amine group may be a primary, secondary or tertiary amine group, preferably a primary or secondary amine group, most preferably a primary amine group.

When the amine is a hydrocarbon substituted with one or more amine groups, the hydrocarbon is optionally further substituted, and the hydrocarbon group may be selected from an alkyl group (including a cycloalkyl group), an alkenyl group (including a cycloalkenyl group), an alkynyl group, an aryl group. , alkaryl, aralkyl and alkaryl. The hydrocarbon portion can be linear or branched. Preferably, the hydrocarbon moiety is an optionally substituted alkyl (including cycloalkyl), aryl, alkaryl, aralkyl or alkarylalkyl group which may be straight or branched.

When the amine is a hydrocarbon substituted with one or more amine groups, the hydrocarbon is optionally further substituted, and the hydrocarbon group is typically a C ₂ to C ₃₀ hydrocarbon. For example, a hydrocarbon can include at least 3, at least 5, at least 7, or at least 10 carbon atoms. It may include 30 or fewer, 25 or fewer, 20 or fewer or 15 or fewer carbon atoms.

When the amine is a hydrocarbon substituted with one or more amine groups, the hydrocarbon is optionally further substituted, typically one, two, three, four or more hydrogen atoms of the hydrocarbon are replaced by other functional groups. Suitable functional groups include -OH, -SH, -OR ₃ , -SR ₃ , -Hal, C(O)COR ₃ , -OC(O)R ₃ , -NR ₃ C(O)R ₃ and C(O And NR ₃ R ₃ , wherein each R _{3 is} independently H or a C ₁ to C ₁₀ alkyl or alkenyl group, and wherein each -Hal is independently selected from -F, -Cl, and -Br, such as -Cl. For example, suitable substituent functional groups include -OH, -OR ₃ , -Hal, C(O)COR ₃ , -OC(O)R ₃ , -NR ₃ C(O)R ₃ and C(O) NR ₃ R ₃ , wherein each R _{3 is} independently H or a C ₁ to C ₁₀ alkyl or alkenyl group, and wherein each -Hal is independently selected from -F, -Cl, and -Br, such as -Cl. The substituents are particularly suitable functional groups include -OH and -OR _3, wherein each R ₃ is independently H or a C ₁ to C ₁₀ or a C ₁ to C ₆ alkyl or alkenyl group (e.g., alkyl).

In some embodiments, the hydrocarbon moiety is preferably not further substituted.

A particularly suitable amine is tetrahydrodicyclopentadienediamine (TCD diamine) which has the following structure:

For the avoidance of doubt, the terms cycloalkyl and cycloalkenyl as used herein include alkyl and alkenyl groups, which include at least one ring structure, including fused ring systems and bridged fused ring systems. For the avoidance of doubt, the term aryl as used herein is intended to include fused aromatic ring systems, and the terms alkaryl, aralkyl and alkarylalkyl should be interpreted accordingly.

Alternatively, the amine can be a polymeric amine. Particularly suitable polymeric amines include polyetheramines and copolymers thereof such as polyether monoamines, diamines and triamines. Suitable polyetheramines include Jeffamine® M-600® or XTJ-505®, Jeffamine M-1000® or XTJ-506®, Jeffamine® M-2005® and Jeffamine® M-2070® (available from Huntsman).

Other suitable amines include Armac C ^® , Armenen HT ^® , Armenen HT/97 ^® , Armenen M2HT ^® , Duomeen TDO ^® , Athomeen ^® and Tetrameen T ^® (available from Akzo Nobel).

As mentioned above, the phosphate free acid ammonium salt is typically the reaction product of a phosphate free acid and an amine. As will be understood by those skilled in the art, ammonium salt formation is usually achieved by the free acid group. Deprotonation occurs to form a phosphate anion and a protonated amine to form an ammonium salt.

The phosphate free acid ammonium salt can be prepared by adding the phosphate free acid ammonium salt to the organic medium prior to mixing the phosphate free acid and the amine in a suitable solvent and allowing it to react to form an ammonium salt. Suitable solvents include polar aprotic solvents such as glycol ether solvents, such as diethylene glycol butyl ether. When the phosphate free acid ammonium salt is prepared prior to its addition to the organic medium, it (along with the solvent in which it is prepared) is typically in the form of a gel.

Alternatively, the phosphate free acid and amine can be added directly to the other components of the organic medium (or directly to the paste) without going through the previous reaction to form the ammonium salt. The inventors have found that the rheological behavior of the paste prepared by the two methods is very similar.

Typically, the ratio of phosphate free acid to amine is about 3:1 by weight. The ratio can be at least 0.5:1, at least 1:1, or at least 2:1 by weight. It may be 10:1 or less, 5:1 or less, or 4:1 or less by weight.

The conductive paste may include at least 0.1% by weight of a phosphate free acid ammonium salt. It may comprise at least 0.2 wt%, at least 0.3 wt%, at least 0.4 wt%, at least 0.5 wt%, at least 0.6 wt%, at least 0.7 wt%, or at least 1 wt%. It may include 10 wt% or less, 8 wt% or less, 6 wt% or less, 5 wt% or less, 3 wt% or less, 2 wt% or less, 1.5 wt% or less, or 1.2 wt% or less. The amount of phosphate free acid ammonium salt was calculated by combining the amount of amine supplied to the paste and the amount of phosphate free acid.

The organic medium can comprise at least 0.5% by weight of a phosphate free acid ammonium salt. It may comprise at least 1 wt%, 2 wt%, at least 3 wt%, at least 4 wt%, at least 5 wt%, at least 6 wt%, at least 7 wt% or at least 10 wt% of a phosphate free acid ammonium salt. It may include 75 wt% or less, 50 wt% or less, 30 wt% or less, 20 wt% or less, 15 wt% or less, or 12 wt% or less of a phosphate free acid ammonium salt. The amount of phosphate free acid ammonium salt was calculated by combining the amount of amine supplied to the paste and the amount of phosphate free acid.

As mentioned above, in addition to the phosphate free acid ammonium salt, the organic medium typically also contains an organic solvent which, as appropriate, dissolves or disperses one or more additives. Suitable for organic media Examples of the solvent include one or more solvents selected from the group consisting of diethylene glycol butyl ether, diethylene glycol butyl ether acetate, terpineol, dialkyl glycol alkyl ether (such as two Ethylene glycol dibutyl ether and tripropylene glycol monomethyl ether), ester alcohols (such as Texanol®), 2-(2-methoxypropoxy)-1-propanol, and mixtures thereof.

Examples of suitable additives include those which aid in dispersing the solid portion in the paste, viscosity/rheology modifiers, shake modifiers, wetting agents, thickeners, stabilizers, and surfactants.

For example, the organic medium may comprise one or more selected from the group consisting of rosin (rosin resin), acrylic resin (eg, Neocryl®, such as Neocryl® 723 or Neocryl® 725 or mixtures thereof), poly Alkyl ammonium salts of carboxylic acid polymers (for example, Dysperbik® 110 or Dysperbik® 111), polyamide waxes (such as Thixatrol Plus® or Thixatrol Max®), nitrocellulose, ethyl cellulose, hydroxypropyl cellulose , rosin and lecithin.

In some embodiments, the organic medium preferably consists essentially of a solvent, a phosphate free acid ammonium salt, any residual (unreacted) amine and/or phosphate free acid, and incidental impurities.

Mixed oxide

The solid portion of the conductive paste of the present invention may include 0.1% by weight to 15% by weight of a mixed oxide (for example, glass frit) with respect to the total mass of the solid portion. The solid portion of the conductive paste may include at least 0.5 wt% or at least 1 wt% of a mixed oxide (e.g., glass frit). The solid portion of the conductive paste may include 10 wt% or less, 7 wt% or less, or 5 wt% or less of a mixed oxide (e.g., glass frit).

Typically, the mixed oxide (e.g., glass frit) will have a softening point in the range of from 200 °C to 400 °C. For example, the mixed oxide may have a softening point in the range of from 250 °C to 350 °C. The softening point can be determined, for example, according to standard ASTM E1356 "Standard Test Method for Assignment of the Glass Transition Temperature by Differential Scanning Calorimetry" using DSC measurement.

The particle size of the mixed oxide powder (for example, glass frit) is not particularly limited in the present invention. Typically, the D50 particle size can be at least 0.1 μm, at least 0.5 μm, or at least 1 μm. The D50 particle size may be 15 μm or less, 10 μm or less, 5 μm or less, 4 μm or less, 3 μm or less, or 2 μm or less, or 1 μm or less. The particle size can be determined using a laser diffraction method such as a Malvern Mastersizer 2000.

Typically, the mixed oxide is a glass frit. Typically, the glass frit is prepared by mixing the raw materials together and melting them to form a molten glass mixture followed by quenching to form a glass frit. The process can further comprise grinding the glass frit to provide the desired particle size.

The composition of the mixed oxide (for example, glass frit) is not particularly limited in the present invention. Typically, the mixed oxide (eg, glass frit) contains ruthenium and may further comprise lead. It can be a glass powder based on lead and antimony. Those skilled in the art are well aware of glass powders suitable for use in conductive pastes for photovoltaic applications.

Conductive metal

The solid portion of the conductive paste of the present invention may comprise from 75 wt% to 99.9 wt% of the conductive metal relative to the total mass of the solid portion. For example, the solid portion can include at least 75 wt%, at least 80 wt%, at least 85 wt%, at least 90 wt%, at least 93 wt%, or at least 95 wt% of the conductive metal. The solid portion may include 99.9 wt% or less, 99.5 wt% or less, or 99 wt% or less of the conductive metal.

The conductive paste may include 75% by weight to 95% by weight of the conductive metal with respect to the total mass of the conductive paste. For example, the conductive paste can include at least 75 wt%, at least 80 wt%, at least 82 wt%, at least 84 wt%, or at least 85 wt% of a conductive metal. The conductive paste may include 99 wt% or less, 95 wt% or less, 90 wt% or less, 87 wt% or less, or 86 wt% or less of a conductive metal. As demonstrated in the examples below (particularly with reference to Example 5), excellent performance was unexpectedly observed even for pastes comprising a reduced amount (e.g., about 85 wt%) of conductive metal.

The conductive metal may comprise one or more metals selected from the group consisting of silver, copper, nickel, and aluminum. Better Ground, the conductive metal contains or consists of silver.

The conductive metal can be provided in the form of metal particles. The form of the metal particles is not particularly limited, but may be in the form of flakes, spherical particles, particles, crystals, powders or other irregular particles or a mixture thereof.

The particle size of the conductive metal is not particularly limited in the present invention. Typically, the D50 particle size can be at least 0.1 μm, at least 0.5 μm, or at least 1 μm. The D50 particle size may be 15 μm or less, 10 μm or less, 5 μm or less, 4 μm or less, 3 μm or less, or 2 μm or less. The particle size can be determined using a laser diffraction method (for example using a Malvern Mastersizer 2000).

The solid portion may include one or more additional additive materials, for example, from 0 wt% to 10 wt% or from 0 wt% to 5 wt% of additional additive material relative to the total mass of the solid portion.

Light receiving surface electrode and solar cell manufacturing

Those skilled in the art are familiar with suitable methods of fabricating light receiving surface electrodes for solar cells. Similarly, those skilled in the art are familiar with suitable methods of making solar cells.

A method of fabricating a light-receiving surface electrode of a solar cell generally comprises applying a conductive paste onto a surface of a semiconductor substrate and firing the coated conductive paste.

The conductive paste can be applied by any suitable method. For example, the conductive paste can be applied by printing, such as screen printing.

Those skilled in the art will be aware of suitable techniques for firing coated conductive pastes. A typical firing process lasts for about 30 seconds with the surface of the light receiving surface electrode reaching a peak temperature of about 800 °C. Typically, the furnace temperature will be higher to achieve this surface temperature. Firing can last, for example, for 1 hour or less, 30 minutes or less, 10 minutes or less, or 5 minutes or less. The firing can last for at least 10 seconds. For example, the peak surface temperature of the light receiving surface electrode may be 1200 ° C or lower, 1100 ° C or lower, 1000 ° C or lower, 950 ° C or lower, or 900 ° C or lower. The peak surface temperature of the light receiving surface electrode may be at least 600 °C.

The semiconductor substrate of the light receiving surface electrode may be a germanium substrate. For example, it may be a single crystal semiconductor substrate or a polycrystalline semiconductor substrate. Alternative substrates include CdTe. Semiconductor For example, it is a p-type semiconductor or an n-type semiconductor.

The semiconductor substrate may include an insulating layer on its surface. Generally, the conductive paste of the present invention is coated on top of an insulating layer to form a light receiving surface electrode. Typically, the insulating layer will be non-reflective. Suitable insulating layers are SiNx (eg SiN). Other suitable insulating layers include Si ₃ N ₄ , SiO ₂ , Al ₂ O ₃ and TiO ₂ .

A method of manufacturing a solar cell generally comprises applying a backside conductive paste (for example, comprising aluminum) to a surface of a semiconductor substrate, and firing a backside conductive paste to form a back electrode. The back conductive paste is usually applied to the surface of the semiconductor substrate opposite to the light receiving surface electrode.

Usually, a back surface conductive paste is applied to the back surface (non-light receiving surface) of the semiconductor substrate and dried on the substrate, after which the front surface conductive paste is applied to the front surface (light receiving surface) of the semiconductor substrate and dried on the substrate. . Alternatively, the front side paste can be applied first, followed by the back side paste. The conductive paste is usually co-fired (i.e., fired with a substrate coated with a front side paste and a back side paste to form a solar cell including front and back conductive tracks).

The efficiency of a solar cell can be improved by providing a passivation layer on the back side of the substrate. Suitable materials include SiNx (eg, SiN), Si ₃ N ₄ , SiO ₂ , Al ₂ O _{3 ,} and TiO ₂ . Typically, the area of the passivation layer is partially removed (eg, by laser ablation) to permit contact between the semiconductor substrate and the backside conductive track.

When a range is specified herein, the endpoints of the range are intended to be independent. Thus, each of the above-described endpoints that are expressly contemplated can be independently combined with each of the lower endpoints, and vice versa.

Instance Paste preparation

A paste having the composition specified in Table 1 below was prepared:

(All compositions are given in wt%; CE means comparative examples.)

Dispersant: Tallicin®

Rheology Modifier: Thixatrol Max®

Acrylic resin: a mixture of Neocryl® 725 and Neocryl® 723

Glass powder: glass powder based on lead and antimony.

By mixing a phosphate free acid (Lubrizol® 2062) with tetrahydrodicyclopentadienediamine (TCD diamine) in a diethylene glycol butyl ether solvent, and allowing a reaction between the phosphate free acid and the diamine The phosphate free acid ammonium salt is prepared by producing an ammonium salt in a diethylene glycol butyl ether solvent. The reaction product has a gel form. The components are mixed in the ratios shown in Table 2 below to produce a gel:

This gel is then mixed with the other components of the paste. In the composition of Table 1 above, the diethylene glycol butyl ether of the gel additive is included in the total amount of the specified diethylene glycol butyl ether. The amount of phosphate free acid ammonium salt is specified in terms of the amount of Lubrizol® 2062 and TCD diamine starting material which react to form the phosphate free acid ammonium salt.

(It should be noted that the inventors have discovered that an alternative method of preparing a paste provides a paste having printability and rheological properties that are very similar to the printability and rheological properties reported herein, in which the alternative method The phosphate ester free acid and amine are added directly to the paste without first reacting it to form an ammonium salt.)

A paste having the composition specified in Table 3 below was prepared:

(All compositions are given in wt%; CE means comparative examples.)

Rheology modifier: CLiQFLOW® 709

Glass powder: based on enamel glass powder

In Comparative Example 2 and Example 6, the phosphate free acid and the amine were directly added to the paste without first reacting it to form an ammonium salt.

Paste printing

The paste is screen printed onto the wafer. Figure 1 shows a printed line using the paste of Comparative Example 1. 2 to 5 show printed lines using the pastes of Examples 1 to 5, respectively. 8 and 9 show printed lines using the pastes of Comparative Example 2 and Example 6, respectively. The results show that due to the inclusion of the phosphate free acid ammonium salt, fewer traces are formed at the edges of the lines of the examples. The results also show that when a larger amount of phosphate free acid ammonium salt is included, less stain is formed at the edges of the lines.

Rheological test

The turbid flow variables of the pastes of Comparative Example 1, Example 4, and Example 5 were measured. The shaking experiment was performed using a rheometer (Model Discovery HR-2) from TA Instruments under the following conditions: rotor: 20 mm plate.

Method: 3 steps measurement

Step 1: Adjust the sample at -25C

Step 2: Frequency 1Hz, strain 4.0e-3% to 100%

Step 3: Adjustment - End of test

Figure 7 reports the scan strain measurement of the paste by plotting the strain (x-axis) versus tan δ (y-axis). A tan δ value of less than 1 is preferred. Figure 7 clearly indicates that the paste of Example 4 (609lm) and Example 5 (619lm) has a higher structure than the paste of Comparative Example 1 (jmfs005) because the fracture of the paste structure occurs at a higher oscillation strain (Figure 7 ). In other words, the tan δ value exceeds 1 at a higher oscillation strain. It is believed that this is the reason for the improved screen printing behavior observed for the paste of the present invention.

Solar cell performance test

Commercially available aluminum pastes were printed on the back side of a polycrystalline wafer having a sheet resistance of 90 Ohm/sq and a size of 6 Å, dried in an IR mass belt dryer and randomly divided into 3 groups of 15 batteries. Each of these groups was printed with a front silver paste screen prepared as described above. A first set of 15 cells was printed with the front silver paste screen of Comparative Example 1, the second set was printed with the front silver paste screen of Example 4, and the third set was printed with the front silver paste screen of Example 5.

Commercially available aluminum pastes were printed on the back side of a polycrystalline wafer having a sheet resistance of 90 Ohm/sq and a size of 6 Å, dried in an IR mass belt dryer and randomly divided into 2 groups of 10 batteries. A first set of 10 cells was printed using the front silver paste screen of Comparative Example 2, and a second set of 10 cells was printed using the front silver paste screen of Example 6.

The screen for the front paste has a finger opening of 40 μm. After printing the front side, the cells were dried in an IR mass belt dryer and fired in a Despatch belt boiler. The Despatch boiler has six firing zones with upper and lower heaters. The first three regions were programmed to be about 500 ° C for burning the adhesive from the paste, the fourth and fifth regions were at a higher temperature, and the last region had a maximum temperature of 945 ° C (furnace temperature). The boiler speed of this experiment was 610 cm/min. By using thermocouple measurements during the firing process too The temperature of the surface of the solar cell is used to determine the recorded temperature.

After cooling, the fired solar cells were tested in an I-V curve tracer model from Heltis PV-CTL1 from Halm. The results are shown in Table 4 below. The results shown in Table 4 are provided by the I-V curve tracer by direct measurement or using its internal software calculations. The results shown for Comparative Example 1 and Examples 4 and 5 are the median values of the measured values of the 15 batteries of each paste. The results shown for Comparative Example 2 and Example 6 are the median values of the measured values of the 10 batteries of each paste.

The results indicate that the paste of Example 4 provided a finer line than the paste of Comparative Example 1 due to the higher J _SC value. The paste of Example 5 having a significantly lower conductive metal content surprisingly gave comparable results to the paste of Comparative Example 1. The results further indicate that the paste of Example 6 provided a finer line than the paste of Comparative Example 2 due to the higher _Jsc value.

The fill factor indicates the performance of the solar cell relative to a theoretically ideal (zero resistance) system. The fill factor is related to the contact resistance - the lower the contact resistance, the higher the fill factor will be. However, if the glass paste of the conductive paste is too aggressive, it may damage the pn junction of the semiconductor. In this case, the contact resistance will be low, but due to pn junction damage (recombination effects and lower shunt resistance), a lower fill factor will occur. Thus, a high fill factor indicates a low contact resistance between the germanium wafer and the conductive trace, and firing of the paste on the semiconductor does not adversely affect the pn junction of the semiconductor (ie, the shunt resistance is higher). A fill factor of 77% or higher is desirable.

The quality of the pn junction can be determined by measuring the pseudo fill factor (SunsVocFF). This is a fill factor that is independent of the losses due to resistance in the battery. Therefore, the lower the contact resistance and The higher the SunsVoc FF, the higher the resulting fill factor will be. Those skilled in the art are familiar with methods for determining SunsVoc FF, for example as described in reference 1. The SunsVoc FF is measured under open circuit conditions and is independent of the series resistance effect.

η represents the efficiency of the solar cell and compares the solar input with the electrical energy output. The efficiency of high quality batteries is typically in the range of 17% to 18%. Smaller efficiency changes can be extremely valuable in commercial solar cells.

The open circuit voltage U _OC is the maximum voltage that can be obtained from the solar cell, and this situation occurs at zero current. The open circuit voltage corresponds to the amount of forward bias on the solar cell due to the bias of the solar cell junction and the photogenerated current. If the paste damages the pn junction, the Uoc value can be lowered.

J _SC is the short circuit current density. The short circuit current is the current that passes through the solar cell when the voltage across the solar cell is zero (ie, when it is shorted). The short-circuit current depends on the area of the solar cell, which is why it is provided as current density (in mA/cm ² ). For an ideal solar cell at the optimum resistance loss mechanism, the short circuit current is the same as the photocurrent. Therefore, the short circuit current is the maximum current that can be drawn from the solar cell. A higher J _SC may indicate that a finer line has been printed because it indicates that the battery can generate more current.

By the shunt resistor R _SH is present due to the significant power loss is due to manufacturing defects typically, solar cell design rather poor. By providing an alternate current path for the photo-generated current, the low shunt resistor causes power loss in the solar cell. This diversion reduces the amount of current flowing through the solar cell junction and reduces the voltage from the solar cell. The effect of the shunt resistor is especially severe at low light levels because there will be less photo-generated current. Therefore, this current to shunt loss has a large impact. In addition, the parallel resistance has a greater influence at a lower voltage when the effective resistance of the solar cell is higher.

GridResFront is the gate resistance of the front of the solar cell. It is part of the series resistance (Rs) in Ω. The resistance is measured between bus bars on the front side of the solar cell. The higher the gate resistance, the higher the series resistance will be. Series resistor has a straightforward fill factor The impact, and thus the battery efficiency, has a direct impact.

Claims (24)

A conductive paste comprising a solid portion dispersed in an organic medium, the solid portion comprising a conductive metal and a mixed oxide, wherein the organic medium comprises a phosphate free acid ammonium salt. The conductive paste of claim 1, wherein the phosphate free acid ammonium salt is a reaction product of a phosphate free acid and an amine. The conductive paste of claim 2, wherein the phosphate free acid has a molecular weight of at least 2,000. The conductive paste of claim 2 or claim 3, wherein the phosphate free acid has an acid value in the range of 20 to 40. The conductive paste of any one of claims 2 or 3, wherein the phosphate free acid comprises: (i) a phosphate moiety, wherein at least one of the oxygen atoms is attached to the organic moiety, and (ii) the phosphate moiety Wherein at least one of the oxygen atoms of the phosphate moiety forms an -OH group, wherein the phosphate moieties (i) and (ii) may be the same or different phosphate moieties. The conductive paste of claim 4, wherein the phosphate free acid comprises: (i) a phosphate moiety, wherein at least one of the oxygen atoms is attached to the organic moiety, and (ii) a phosphate moiety, wherein the phosphate moiety At least one of the oxygen atoms forms an -OH group, wherein the phosphate moieties (i) and (ii) may be the same or different phosphate moieties. A conductive paste according to claim 5, wherein the organic moiety is a C ₅ to C ₄₀ hydrocarbon moiety which is optionally substituted. The requested item of the conductive paste 6, wherein the organic moiety of the optionally substituted C ₅ to C ₄₀ hydrocarbon moiety. The conductive paste of claim 5, wherein the amine is a C ₂ to C ₃₀ hydrocarbon substituted with one or more amine groups, the hydrocarbon being further substituted as appropriate. The conductive paste of claim 6, wherein the amine is a C ₂ to C ₃₀ hydrocarbon substituted with one or more amine groups, the hydrocarbon being further substituted as appropriate. The conductive paste of claim 9, wherein the hydrocarbon is substituted with 2 to 5 amine groups. The conductive paste of claim 10, wherein the hydrocarbon is substituted with 2 to 5 amine groups. The conductive paste of claim 5, wherein the amine is a polyether amine. The conductive paste of claim 6, wherein the amine is a polyether amine. A method for producing a conductive paste according to any one of claims 1 to 14, wherein the method comprises mixing together components of a conductive metal, a mixed oxide and an organic medium in any order, wherein the organic medium comprises Phosphate free acid ammonium salt. The method of claim 15, wherein the phosphate free acid ammonium salt is prepared by reacting a phosphate free acid with an amine prior to adding it to the organic medium. The method of claim 15, wherein the phosphate free acid and the amine are provided to the organic medium, and then allowed to react to form a phosphate free acid ammonium salt. A light-receiving electrode for a solar cell, the light-receiving electrode comprising a conductive track on a semiconductor substrate, wherein the paste obtained by firing any one of claims 1 to 14 is obtained or obtainable on the semiconductor substrate Conductor rail. A solar cell comprising a light receiving electrode as claimed in claim 14. A printing method comprising applying a paste composition to a substrate by screen printing, wherein the paste composition comprises a solid portion dispersed in an organic medium, and wherein the organic medium comprises a phosphate free acid ammonium salt. The method of claim 20, wherein the substrate is a semiconductor substrate, and the paste composition is a conductive paste, wherein the solid portion comprises a conductive metal and a mixed oxide. The method of claim 20, wherein the substrate is a glass surface, and the paste is an enamel composition, wherein the solid portion comprises a pigment and a mixed oxide. Use of the conductive paste according to any one of claims 1 to 14 in the manufacture of a light-receiving electrode for a solar cell. Use of a conductive paste according to any one of claims 1 to 14 in the manufacture of a solar cell.